VERTEBRATES. 461 



in the tubercles along the posterior angles than in those occupying the 

 body of the spine. It is possible these differences are merely charac- 

 teristic of age, but as yet our materials are insufficient for the successful 

 investigation of the development of these defensive spines. The rela- 

 tively narrower and more strongly curved outline, and laterally elongated, 

 sharp-crested tubercles distinguish X. arcuatus, Leidy, from the present 

 form. 



Geological position and locality : Discovered by Mr. Ales. Butters, 

 in the carbonaceous roof-shales of coal No. 5, associated with Edestus 

 Heinrichsii, IS. and W., Tetrodus occidentalis, N. and W., etc., Oarlin- 

 ville, Illinois. 



Gews EEISM ACANTHUS, McCoy. 



Eeisjviaoanthus McUotanus, St. J. and W. 



pi. xsrr, Fig. 1-5. 



Ein-spine of medium size. Basal shaft nearly vertical or at right 

 angles to the anterior and posterior arms, with a slight forward direc- 

 tion, very compressed, apparently in part at least due to pressure, the 

 •walls of the inserted portion being very thin and readily yielding to 

 external compression, bluntly rounded interiorly, gently concave poste- 

 riorly, and correspondingly convex along the thickened anterior edge, 

 which was exposed far down its extent, and terminated in a projecting 

 shoulder, beneath which the true base is slightly notched and obliquely 

 truncated something after the fashion of Physonemus ; line of insertion 

 gently and regularly arched from the anteroinferior shoulder to the 

 angle of the posterior basal margin and posterior arm. Anterior 

 spine long, very gradually diminishing towards the apparently obtuse 

 extremity* more or less circular or oval in transverse section in the 

 anterior and middle region, compressed near its origin, and marked by 

 numerous relatively large, depressed or subconical tubercles, disposed 

 in somewhat obscure oblique, crowded rows, overspreading the lateral 

 surfaces of the inferior portion or shaft, where they are relatively small 

 and even less regularly arranged, and extending along perhaps a third 

 of the posterior margin of the posterior spine, those along the anterior 

 (inferior) side of the anterior spine being somewhat produced tubercles, 

 "whose eccentric apices are directed forward, and which constitute a row 

 extending to the anteroinferior shoulder, but along the inferior edge 

 they are much less prominent. The posterior arm consists of a rela- 

 tively short spine, about one-third the length of the anterior arm, rap- 

 idly tapering, moderately curved, laterally compressed, low-inclined, 

 anterior margin thickest, and forming an obtuse angle at the point of 



